Upgrade MyGo to v0.5.5

MyGo v0.5.5 returns ui.Element as a checked value handle instead of a
pointer. Migrated with `mygo migrate-ui`, then fixed what the codemod
leaves to review:

- Element nil checks become .Valid(); absent elements are ui.Element{}.
- Input handlers and paint callbacks run outside the build pass, where
  elements expire and Context methods are inert: read IsDisabled into a
  local during the build, keep a ui.Handle for Focus, use c.Services()
  for clipboard access, create ui.Local state during the build.
- Add core.AutoFocus: v0.5.5 records an overlay's opener after its
  content builds, so same-frame autofocus lost focus restore on close.
  It focuses one frame later; cascader rows defer their focus likewise.
- core.OverlayLayer identifies build passes by the root element, not
  c.Now(), which repeats across passes and under the Tester clock.
- Context menus survive the opening press counted as PressedOutside;
  split button tooltip no longer reorders its open menu; disabled
  menubar rows pass presses to the panel; Form defers blur-started
  async checks so the growing message no longer moves Submit away
  from the release; Combobox drops its disabled copy now that MyGo
  blocks typing under a disabled wrapper (canary flipped to a guard).
- The Tester clock is virtual: tests use Advance/Now instead of
  time.Sleep/time.Now and read element state inside the view. The web
  mirror advances the clock to wall time before each frame and input.
- AGENTS.md and docs describe the value Element API and the new pin.

Snapshot goldens are darwin-only and were not re-verified here.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

chat/markdown_view.go

@@ -13,7 +13,7 @@ import (
 
 // MarkdownResult is a MarkdownView's element and the link the user opened this frame.
 type MarkdownResult struct {
-	Element *ui.Element
+	Element ui.Element
 	link    string
 }
 
@@ -32,7 +32,7 @@ func MarkdownView(c *ui.Context, src string) MarkdownResult {
 	r.Element.Children(func() {
 		for i := range (*doc).Blocks {
 			b := &(*doc).Blocks[i]
-			ui.Column(c).Key(i).MinWidth(0).Children(func() {
+			ui.Column(c.Key(i)).MinWidth(0).Children(func() {
 				if url := markdownBlock(c, b); url != "" {
 					r.link = url
 				}
@@ -96,7 +96,7 @@ func markdownBlock(c *ui.Context, b *internal.MdBlock) string {
 }
 
 // markdownSpans fills paragraph t with styled spans; links are focusable and return their URL when used.
-func markdownSpans(c *ui.Context, t *ui.Element, spans []internal.MdSpan) string {
+func markdownSpans(c *ui.Context, t ui.Element, spans []internal.MdSpan) string {
 	k := core.Tokens(c)
 	var link string
 	t.MinWidth(0).Children(func() {